(trad, set): Treat trade file slots with zero owner as unused, so that

(trad, set): Treat trade file slots with zero owner as unused, so that
freshly extended trade file makes sense.  When set() removes a lot,
simply zero the slot.

(set): Initialize trd_uid.

(trade_check_ok): Use check_land_ok() & friends instead of doing it by
hand, but wrong.  This fixes bying planes and land units loaded on
ships that moved after loading.

(check_trade_ok): New.
(trade_check_ok): Use it.  Remove unused argument lot.  Callers
changed.